Recent years have seen the development of electronic ophthalmological devices that utilize laser scanning. Such instruments are called scanning laser ophthalmoscopes (which herein shall be abbreviated as "SLO"), and particular progress has been made with such devices for use in observing the eye fundus (described, for example, in JP-A-62-117524, USP 4764005; and JP-A-64-58237, U.S. Pat. No. 4854692).
With a conventional SLO, by scanning the eye fundus two-dimensionally by a laser beam passing through the center of the pupil, and receiving, photodectrically converting and amplifying the light reflected by the eye fundus from a larger area around the pupil, it is possible to display a real-time video image of the eye fundus on a TV monitor with a low brightness and a high S/N ratio. Also, the effect of non-essential scattered light can be screened out by using an optical system with a confocal arrangement system, thereby producing a marked improvement in fundus image resolution and contrast.
Among other advantages provided by these new instruments that are producing major innovations in ophthalmology are that they make it possible to decrease greatly the amount of fluorescent agent that needs to be administered for fluorescent angiography of the eye fundus, and by modulating the scanning laser beam it becomes possible to examine visual function while observing the eye fundus, and the monochromatic properties of lasers make it possible to carry out precision examination of the fundus, and real-time three-dimensional observation (cf. JP-A-1-113605, U.S. Pat. No. 4,900,144, Optics Communications, vol. 87, 1992, pp 9-14).
To obtain the large amounts of information required for diagnoses using such ophthalmic examination apparatuses, it is desirable to prepare a plurality of laser light sources of different wavelengths. For example, infrared, red, yellow, green, blue and other wavelengths are used for different image observation applications, as well as, in the case of angiography, fluorescein angiography (FAG) and indocyanine green angiography (ICG) and the like.
To meet the advanced needs of ophthalmologists, this means that it is necessary to prepare three or four laser light sources and to incorporate all of them in the optical system of the apparatus. This results in an increase in the size of the optical system (optical head): an example of one such apparatus is illustrated on page 444 of "Noninvasive Diagnostic Techniques in Ophthalmology," (Springer, Berlin, 1990).
During use of the foregoing conventional of ophthalmological apparatus, the patient is seated on a chair, facing the optical head, which is moved vertically and horizontally to bring it into alignment with the optical axis of the eyeball. In a clinical setting, a large optical head is undesirable because it makes the machine more difficult for the ophthalmologist to operate and also causes the patient unnecessary anxiety.
In view of the large size of laser light sources and the heat they generate, especially short-wavelength, visible-light gas lasers, it is generally preferable for the laser tube to be located away from the optical head, which has led to the use of a flexible optical fiber to link the laser light source and optical system, as described by JP-A-60-132536, JP-A-1-101959, and JP-A-3-63303, for example. JP-A-3-63303 even describes the use of an optical fiber connection between the confocal aperture of the optical system and the light receiving element, with the aim of decreasing the size of the optical head.
The diversification of application needs and structural complexity are problems associated with conventional ophthalmic examination apparatuses (SLO). The laser source used by an ophthalmologist conducting an examination of the eye fundus will vary depending on the ailment involved. Thus, one source might be used to check for diabetic degeneration, another to examine ophthalmic circulation, for example. That is, use of the most diagnostically effective wavelength will differ from ailment to ailment. Even when only single wavelength capability is initially specified, advances in clinical research often make it necessary at a later date to expand the laser source capabilities or upgrade a system's image processing performance, and manufacturers have to be able to respond to such individual user requirements.
Conventional ophthalmic examination apparatuses equipped with a single wavelength laser cannot meet such diversifying needs. On the other hand, initially configuring the apparatuses with a multiplicity of lasers, including image processing functions, increases the size and the cost and makes them more difficult to maintain and expensive to transport, There, such apparatuses are unable to answer the needs of high-level medical treatment, in terms of both economics and efficiency.
Another drawback with conventional ophthalmic examination apparatuses, especially those which use an optical fiber connection between the laser light source and the optical system, is the of degradation of picture quality. As the aim is to obtain an image of the eye fundus or other such part of interest by two-dimensional scanning with a laser beam, in order to improve the resolution it is necessary to reduce the size of the scanning light spot. As such, it is advisable to use a single-mode optical fiber having a core diameter no larger than 10.mu.m.
However, when the beam from the laser tube is tightly focused onto the end face of the optical fiber to effect the link, light reflected from the end face of the fiber propagating back to the laser tube can destabilize the lasing action and cause the power output to fluctuate.
How this happens, in the case of a gas laser using a round tube, is illustrated by FIG. 6. A laser beam 61a emitted from a laser tube 601 in the laser light source 61 is linked to an optical fiber 65 by a connector 64.
To increase the transmission efficiency of an optical fiber, especially a single-mode fiber, the laser beam has to be focussed precisely onto the center of the core.
However, with the beam aligned precisely with the core center for maximum transmission efficiency, a return component 603 from a slight reflection from the fiber end face 602 travels back along the path of beam propagation and into the laser tube 601. It is as if a composite resonator has been formed between resonator mirrors (M1) 604 and (M2) 605 and the fiber end face 602, and can produce extreme instability of the lasing action and a major degree of fluctuation noise.
Another problem encountered when an optical fiber is used to transmit the laser beam is that movement or vibration of the fiber can cause beam power variation. Although conventional ophthalmic devices such as laser coagulators and the like use an optical fiber to transmit the laser beam, such power variation is not a significant problem, either because a large-core multimode fiber is used or because slight variations in beam power are not a hindrance to the objective of fixating the fundus.
However, with SLOs and other such imaging systems which are required to provide images with a high S/N ratio, the variations in beam power resulting from the conventional use of optical fibers gives rise to noise which is superimposed on the TV pictures, degrading the picture quality, which is a problem particularly when viewing images on a real-time basis.
The object of the present invention is to provide an ophthalmic examination apparatus that can answer the diverse needs of clinical ophthalmology without increasing its size and cost and is easy to maintain and transport, and which provides good images of the region being examined, with no degradation in picture quality even when the laser light source and optical system are linked by an optical fiber.